1. Field of the Invention
The present invention relates to a light diffusion film having the function of absorbing ultraviolet light, and more particularly, to a light diffusion film for use in a surface light source unit for a liquid crystal display or the like, to a surface light source unit using the light diffusion film, and to a liquid crystal display using the surface light source unit.
2. Background Art
Since liquid crystals themselves do not emit light, liquid crystal displays for personal computers, word processors, liquid crystal televisions, and the like are illuminated from their rear by surface light source units (also referred to as backlight units). The surface light source units for uniformly illuminating the entire liquid crystal display screens are classified into the edge light type using a light guide plate and the direct type using fluorescent lamps arranged in parallel. In a surface light source unit of edge light type, light from a line light source enters a light guide plate from its edge face and emerges from the surface of the light guide plate via a light-scattering pattern provided on the back surface of the light guide plate. Thus, the line light source is converted into a surface light source. In the case of a surface light source unit of direct type, a plurality of line light sources are arranged right under a liquid crystal display to illuminate the liquid crystal display screen. Such a surface light source unit comprises a light diffusion film, a lens film, a protective light diffusion film, etc. to let the light from the light source emerge from the plane of emergence, to diffuse or converge the emerging light, and to make the luminance on the illuminated surface uniform.
In recent years, liquid crystal displays have been made large in screen size and have come to be used for televisions (TVs), and so forth. Although the luminance of images displayed on conventional small screens has not been a significant matter, TVs are required to attain high luminance and wide viewing angles because many people watch a TV at the same time in a relatively brightly lighted room. Therefore, a direct-type surface light source unit using, as a light source, a plurality of fluorescent lamps with high output is mainly used for a liquid crystal display for a TV.
However, when the output of this light source is increased, radiation of unwanted ultraviolet light increases along with radiation of visible light. Consequently, optical members, such as a light diffusion film, a lens film, and a polarizer film, contained in the surface light source unit undergo deterioration by the ultraviolet light and are colored or cracked. A possible way to avoid such deterioration, of course, is to add ultraviolet light absorbers to the lens film, the polarizer film, and so on. However, the optical films such as the polarizer film and the lens film are required to have the desired optical parameters such as polarization characteristics and refractive index, so that the addition of impurities such as ultraviolet light absorbers has been detrimental and also laborious. Further, the addition of ultraviolet light absorbers to resins such as acrylic resins that are usually used for the optical films has also been disadvantageous in that the ultraviolet light absorbers bleed off from the resins with time, which leads to decrease in the amount of the ultraviolet light absorbers, and that the ultraviolet light absorbers that have bled off the resins are crystallized to make the resins cloudy.